Is blender good for real physics simulations?
All physics simulations are approximations :-)
It just depends on how accurate you need it to be. As for your example, a box sliding on a slope is in reality quite complex. You do not only have imperfect, uneven surfaces, but they will also deform under load, maybe even be wet in places, they will possibly abrade and so on.
Depending on the level of accuracy you need, you can either use the fast but somewhat inaccurate blender physics, some third-party multibody simulation tool, or even a full FEM-analysis.
In any way, there are possibilities to use blender for visualization of the results afterwards.
Maybe just describe a little more in detail, what physical effects you want to show and then someone can point you in the right direction!